Each type of electrochemical cell has a characteristic "full charge" voltage. A lower cell voltage indicates a state of charge less than "full". To obtain a higher voltage than can be provided from a single cell, cells are connected in series, often with internal or integral connections between the cells, to form a battery which has the desired level of output voltage. Certain types of electrochemical cells, such as rechargeable alkaline manganese, lithium, and lithium ion cells have no internal charge control mechanism. Consequently, if charging of such cells is not carefully controlled, over-charge can result, which will cause irreversible changes in cell chemistry, performance loss, and, in extreme cases, cell venting. The charging of series connected cells of these types is therefore difficult, since cell voltages and capacities may not be equal, with the result that some cells are overcharged.
To address this problem, equalizers have been developed which monitor the voltage across each cell and connect resistors or current sinks across the cell or cells having an excessive voltage to partially discharge the cell and thereby accomplish charge equalization among the cells. However, equalizing the charge on cells in this manner wastes power and also causes undesirable heating of the battery pack since the equalization circuitry is typically physically located in the battery housing. In addition, the rate at which the cells can be equalized, and thus the rate at which the cells can be recharged, is limited to the amount of power dissipation that can be tolerated.